With the popularization of the Internet, the capacity of optical communication systems has been increasingly growing. By way of example, for a trunk line system, an optical transmitter and an optical receiver which can respectively transmit and receive a signal exceeding 40 Gbit/s per wavelength are currently under study.
If the bit rate per wavelength is increased, signal quality is significantly degraded by a decrease in an optical signal-to-noise ratio (OSNR) tolerance and any waveform distortion caused by the chromatic dispersion of a transmission line, polarization mode dispersion, nonlinear effects, etc. Accordingly, attention has been focusing on a digital coherent reception system with high OSNR tolerance and waveform distortion tolerance in recent years.
In the digital coherent reception system, optical intensity information and phase information are extracted from a received signal, which is then demodulated by a digital signal processing circuit. Also in the digital coherent reception system, an OSNR tolerance is improved with coherent reception, and a waveform distortion is compensated by the digital signal processing circuit. Therefore, high reliability can be achieved even in an optical communication system exceeding 40 Gbit/s. A technique for receiving QPSK optical signal with coherent detection is recited, for example, in D. Ly-Gagnon et al, “Coherent Detection of Optical Quadrature Phase-Shift Keying Signals With Carrier Phase Estimation”, IEEE Journal of Lightwave Technology, vol. 24, No. 1, pp 12-21, January 2006.
Additionally, a configuration for transmitting specific information that is different from a data signal between a transmitter and a receiver in an optical communication system is known. For example, monitor information or control information is transmitted between the transmitter and the receiver, whereby improvements in a transmission characteristic, the establishment of a flexible network, and cost reductions in communication appliances can be expected.
In the transmission system of a related art, a transmitter includes an output control unit for adjusting a output signal level. The output control unit controls the output signal level of each channel according to feedback information from a receiver so that the optical intensity of each channel in the receiver attains a certain level. The configuration of the related art is described, for example, in Japanese Laid-open Patent Publication No. 05-292033.
In the transmission system of another related art, a receiver estimates the chromatic dispersion of a transmission line by monitoring a bit error rate, and feeds back the estimation result to a transmitter. The transmitter has a function (pre-distortion function) to add, to a transmission signal, a waveform distortion for compensating for the chromatic dispersion of the transmission line on the basis of a notification made from the receiver. The configuration of the other related art is described, for example, in D. McGhan, “Electronic Dispersion Compensation”, OFC 2006, OWK1, 2006.
As described above, the method by which a control signal is transmitted between a transmitter and a receiver in addition to a data signal in an optical communication system adopting coherent reception has been proposed. With the conventional technology, however, a control signal is generally recovered by using a dedicated circuit in the receiver. Therefore, the circuitry scale of the receiver increases. Otherwise, the use efficiency of wavelength resources is low in a configuration for allocating a dedicated wavelength to a control signal. This problem is not limited to a system that transmits a control signal, and may possibly occur in a system that transmits information items different from a data signal.